That all depends on the construction of the floor, do it badly enough and it won't even support itself.
This depends on the load (s) being applied to the bridge. Once an Engineer has determined the forces and loads that will act upon a bridge then he can start the design process selecting the proper...
14lbs. try it.
It depends on the density (the mass per unit volume) not just the weight. That is why a one ton anchor sinks while a 100,000 ton ship floats.
it depen on the size of the joist